1. Field of the Invention
The present invention relates to a solution for a catalytic treatment, and more specifically, to a solution for a catalytic treatment for use in adhering, to a substrate, metals serving as catalysts during an electroless plating or catalyst-metals involved in various catalytic reactions. The present invention also pertains to a method of applying a catalyst-metal to a substrate while using such a solution for a catalytic treatment, as well as a method of forming an electrical conductor on a substrate.
2. Description of the Related Art
Among the catalysts used in conventional electroless plating, most generally used is a Pd (palladium)-Sn (tin) colloid. Pd, however, serves only as a catalyst for the plating reaction, and Sn merely promotes the adhesion of Pd to a surface to be plated and often adversely affects the progress of the plating. For this reason, the plating is usually performed after adsorbing the Pd-Sn colloid to a substrate and then subjecting the substrate to an "Sn-extraction" treatment with an accelerator.
FIG. 1 shows the sequence of the processes of an example of a conventional electroless plating in which such a Pd-Sn colloid is employed as a catalyst:
First, as shown in FIG. 1A, there is provided a substance 1 to be plated; the surface 10 thereof to be plated having been cleaned. Then the substance to be plated is immersed in a Pd-Sn colloidal solution to apply the Pd-Sn colloid as a catalyst to the surface to be plated. The substance is then withdrawn from the colloidal solution to produce a substrate 1 to which Pd is adhered or adsorbed to the surface to be plated through an Sn layer surrounding the Pd particle or atom as shown in FIG. 1B. As mentioned above, the presence of Sn may adversely affect the plating reaction, and therefore, the substance to be plated is immersed in an appropriate accelerator solution for carrying out the Sn-extraction treatment, to thus remove any excess Sn as shown in FIG. 1C. After completion of the Sn-extraction treatment, the electroless plating is performed in the usual manner. The Pd nuclei effectively serve as a catalyst, and thus a plated film 21 is formed as shown in FIG. 1D.
This conventional method, however, suffers from some problems, most serious of which is that Pd-Sn is adsorbed, through the immersion, to the portions of the substance which should not be plated, and therefore, the catalyst is also present on such portions. This problem can be solved by forming a mask on the portions which should not be plated, with a resist or the like covering the catalyst layer formed, but the perpetual presence of the catalyst between the resist layer and the substrate becomes a cause of a deterioration of the electrical properties of the resulting articles. In addition, most of the Sn can be removed through the Sn-extraction process, but the substance to be plated is necessarily contaminated with Sn, and thus the plating reaction is sometimes interrupted. These two problems become serious when a more precise plating is intended.
A method in which a catalyst-metal palladium is directly deposited on the surface to be plated, without using Sn, is disclosed in, for example, Japanese Unexamined Patent Publication (hereinafter referred to as "KOKAI") No. 61-127868. In this method, metal palladium nuclei are applied to the surface by the vapor phase growth technique. Therefore, the use of a large-scale apparatus is necessary and there is still room for an improvement of the reliability and adhesion of a plated film to a substrate.
As for the foregoing electroless plating, attention must be paid to the formation of electrical conductor (electrodes, herein) on portions to which a catalyst can be applied only with difficulty. This problem is encountered in, for example, the production of an oxygen (O.sub.2) sensor. This is because, when the electrodes of sensors such as an oxygen sensor are formed, as is well-known, a metal serving as a catalyst, i.e., a catalyst-metal must be applied to the surface of a substrate before plating with a metal to form the electrodes, but in this case, the catalyst-metal must be applied to an inner wall having a cup-like shape, which is very difficult in practice.
More specifically, an O.sub.2 sensor comprises a substrate 31 made from a solid electrolyte having a cup-like shape, electrodes for determination disposed on both external and internal walls thereof, and a reference electrode 32 (Pt) as shown in FIG. 2, and these Pt electrodes are formed through a chemical deposition which comprises, for example, the following steps. To form the electrode, a catalyst (in general, metals such as Pd and Pt are used) serving as a nucleus for the plating must be applied to the surface of the substrate, as described above in connection with the electroless plating. The formation of the catalyst is performed by spraying the external surface of the substrate of the O.sub.2 sensor with a mixture of selected catalyst-metal ions (such as Pd ions or Pt ions) and a reducing agent, and applying ions such as Pd or Pt ions to the internal surface of the substrate, drying for more than about 5 hours to evaporate moisture and adhere the ions to the surfaces, and then reducing the adhered ions to their elemental metal state. Thus, the application of the catalyst to the external surface of the substrate can be performed within a short period of time, but the application of the catalyst to the internal surface thereof is complicated and requires treatments that to be a long time, and this becomes a cause of an increase in the number of steps needed. In addition, this method of applying a catalyst does not permit a selective formation of a catalyst only on the desired portions on which electrodes are to be formed, and thus the entire surface of the substrate including both the external and internal surfaces thereof are covered. Accordingly, a film of expensive Pt must be formed even on portions of the surface of the substrate other than those on which electrodes are to be deposited through a chemical plating method. Namely, this method is also unfavorable from the economical viewpoint.
Under such circumstances, there have been proposed a variety of methods of partially applying a catalyst to the surface of a substrate, to thus permit a selective formation of Pt electrodes. For example, KOKAI No. 61-234351 proposes the use of an adhesive tape. More specifically, the method disclosed in this patent comprises masking, with an adhesive tape, portions of the surface of a substrate other than those on which Pt electrodes are to be formed, applying a catalyst under such a masking condition in a conventional manner, and then peeling off or removing the adhesive tape. In this method, however, the adhesive tape for masking must be applied to each article and although this reduces the cost of the electrode material (such as Pt), the number of steps is increased.